Recently, fiber to the home (FTTH) services in which homes are connected via optical fibers to exchange information using the optical fibers have been expanded. In FTTH services for transmitting various types of information, there is a system for simultaneously transmitting a broadcasting signal and other communications signals in their respective modes using a single optical fiber. In general, in this system, in many cases, the broadcasting signal is an analog signal, a baseband signal, or an optical SCM signal. The characteristics of this system from the point of optical fiber as a transmission medium are as follows:
(i) FTTH is based on a typical double-star PON (passive optical network), and a distribution loss increases.
(ii) In order to transmit an analog signal, a baseband signal, or an optical SCM signal, a receiver needs a high CNR (carrier-to-noise ratio), so that the minimum signal light power of a light receiving unit needs to be greater than that in digital transmission used for communications.
As described above, when analog transmission using intensity modulation is performed for video transmission, high power transmission is needed to compensate for a distribution loss and guarantee a high CNR. However, there was a problem in that only a predetermined intensity of light (SBS threshold power) can be transmitted through the optical fiber although higher power is intended to be transmitted, and the remaining light becomes backscattered light and returns to the incident side, so that transmissible signal light power was limited.
As means for suppressing SBS, there is a technique for changing a dopant concentration and a residual stress in a longitudinal direction (for example, see Patent Document 1). In this technique, by changing the dopant concentration or the residual stress in the longitudinal direction, it is possible to enlarge the Brillouin spectrum and suppress occurrences of SBS. In addition, techniques for giving such a refractive index profile for an optical fiber that SBS can be suppressed, are proposed (for example, see Patent Documents 2 to 5, and 7).
Patent Document 1: Japanese Patent No. 2584151
Patent Document 2: WO 2004/1100406
Patent Document 3: U.S. Pat. No. 7,082,243
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-154707
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2006-184534
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2006-133314
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-154713
Non-Patent Document 1: “Design concept for optical fibers with enhanced SBS threshold” Optics Express, Vol. 13, Issue 14, p. 5338 (July 2005), Andrey
Non-Patent Document 2: “Nonlinear Optical Fibers with Increased SBS Thresholds” OFCINFOEC 2006, OTuA3, Scott Bickham, Andrey Kobyakov, Shenping Li.